Radiation curable materials are used over wide ranges in the field of coatings and printing inks. Usually, such materials can be cured by radiation of a ultraviolet ray or an electron beam, through radical or cationic reaction.
In these curing reactions, cationically curable materials such as an epoxy compound and an oxetane compound do not involve an issue of oxygen inhibition in the curing reaction, which has been seriously problematic for radical polymerization systems. Thus, advances have been made in the study of such cationically curable materials in the field of coatings and printing inks. However, generally speaking, such cationically curable compounds are sensitive to air humidity, which as a result makes it difficult to control the physical properties of the cured product of a printing ink or a coating.
To solve this problem, a technique is known in which a hydroxy-containing mono-oxetane compound such as 3-ethyl-3-hydroxymethyloxetane is included as a reactive diluent, to thereby yield an aqueous cationically curable composition hardly sensitive to humidity that has been so demanded for environmental reasons in recent years (see Patent Document 1).
As disclosed in Patent Document 1, when a hydroxy-containing mono-oxetane compound is used as a reactive diluent in an aqueous cationically curable composition, the sensitivity to humidity can be lowered and the viscosity of the composition can be reduced. Moreover, the compound can function as a compatibilizer for dissolving a water-insoluble and polyfunctional cationically curable compound such as an epoxy compound in water.
However, if this hydroxy-containing mono-oxetane compound is used as a reactive diluent, the curing speed is yet slow because the reactive diluent is monofunctional. Thus, no sufficient curability can be achieved. In addition, the crosslink density after curing decreases and the surface tension of an aqueous coating increases. Therefore, the wettability for nonporous/nonpolar plastic substrates and the adhesiveness to these plastic substrates are inferior.
Patent Document 1: U.S. Pat. No. 6,232,361 (from column 4 line 35 to column 5 line 12)